1. Field of the Invention
This invention relates to photogrammetry. More specifically, the invention is a photogrammetric system and method used in the characterization of a structure.
2. Description of the Related Art
Photographic images are the result of a perspective projection of a three-dimensional (3D) object onto two dimensions (2D). Consequently, two or more photographic images can be reverse engineered to derive the 3D shape of the original object. This process is called photogrammetry, and the solution provides a quantitative relationship between a 3D object and the 2D images acquired by cameras.
While photogrammetry has its roots in the topographic mapping and surveying field, the last two decades have seen close-range photogrammetric techniques developed to support various industrial and research applications. For example, in some areas of aeronautics aeroelastic experimentation to include model deformation and wing twist, photogrammetric measurements have become part of the standard data set.
Accurate photogrammetric measurements require the photographing of high contrast surface features that appear in at least two images. However, many objects commonly measured do not naturally exhibit such features. Traditionally, retro-reflective targets are attached to the object to artificially provide these high contrast features. When illuminated, these targets reflect light directly back to the illuminating source causing the targets to appear very bright in contrast to the background.
Retro-reflective targets work exceptionally well and have very few drawbacks when used on solid structures. However, retro-reflective targets are not suitable for all types of structures. One example is ultra-lightweight inflatable membrane space structures. The attachment of retro-reflective targets to lightweight membrane structures introduces unacceptable effects such as added stiffness and weight.
An alternative to the attachment of retro-reflective targets is to project target patterns onto a structure. While this non-contact target generation method has advantages with respect to retro-reflective targets, target patterns projected with standard techniques require a diffuse or optically rough surface to work efficiently. However, most membrane-based space structures are made from either highly transparent or highly reflective materials. Target patterns projected onto these types of materials result in the generation of images having poor contrast due to lack of diffusely scattered light and the presence of glints and hot-spot specular reflections from the target-pattern projector.
Laser-induced target generation techniques have been proposed and demonstrated that solve the problems described above, but require the addition of laser dye to the membrane during manufacture. This precludes the use of this these techniques with existing structures that do not already contain the laser dye. For new structures that can be manufactured containing laser dye, this these techniques still requires a laser source and associated optics that complicate the measurement process and introduce eye safety issues. In addition, the size, weight and complexity of the laser source may preclude use of this technique in space applications.